Almost all production of Cyanocobolamin, the principal clinical form of vitamin B12 are derived from microorganisms. In the UK, a considerable amount is derived from Pseudomonas dentrificans under carefully controlled sterile conditions. The methods discussed above remain commercially impractical at present and will remain so for some time.
Unless they are commercially fortified, plant food products in general do not contain any Vitamin B12. The only exception is those plants that grow in close proximity to soil bacteria that will cling to the plant concerned and are processed along with the plant.
Everything seems to point to you undergoing treatment for pernicious anaemia. This particular type of anaemia is not really due to a lack of or a deficiency of B12 in your body.
Pernicious anaemia occurs because you lack a specific type of glycoprotein in your stomach contents. This protein is called intrinsic factor. Without this intrinsic factor, your body cannot absorb Vitamin B12, the extrinsic factor, in adequate amounts.
Ironically, there are bacteria in the the gut that can process Vitamin B12 correctly, but they are simply too far down the digestive tract to work - by then the body will have begun to destroy the vitamin.
Deficiency of Cyanocobolamin occurs within 2-3 years, but sometimes even strict vegans can remain symptomless for 20-30 years. Nevertheless, some vegetarians demonstrate neurological symptoms, weakness and other symptoms within a relatively short time.
Some other good sources of Vitamin B12 include ox liver, lambs liver, herring, mackerel, pilchards, kippers, tuna and sardines. Marmite is an excellent source and it also provides another important vitamin usually given alongside vitamin B!2 called folic acid.